Influence of Inorganic Anions on the Chemical Stability of Molybdenum Disulfide Nanosheets in the Aqueous Environment

Chemical stability is closely associated with the transformations and bioavailabilities of engineered nanomaterials and is a key factor that governs broader and long-term application. With the growing utilization of molybdenum disulfide (MoS2) nanosheets in water treatment and purification processes, it is crucial to evaluate the stability of MoS2 nanosheets in aquatic environments. Nonetheless, the effects of anionic species on MoS2 remain largely unexplored. Herein, the stability of chemically exfoliated MoS2 nanosheets (ceMoS2) was assessed in the presence of inorganic anions. The results showed that the chemical stability of ceMoS2 was regulated by the nucleophilicities and the resultant charging effects of the anions in aquatic systems. The anions promote the dissolution of ceMoS2 by triggering a shift in the chemical potential of the ceMoS2 surface as a function of the anion nucleophilicity (i.e., charging effect). Fast charging with HCO3– and HPO42–/H2PO4– was validated by a phase transition from 1T to 2H and the emergence of MoV, and it promoted oxidative dissolution of the ceMoS2. Additionally, under sunlight, ceMoS2 dissolution was accelerated by NO3–. These findings provide insight into the ion-induced fate of ceMoS2 and the durability and risks of MoS2 nanosheets in environmental applications.


INTRODUCTION
With their remarkable attributes, two-dimensional (2D) nanosheets of layered transition metal dichalcogenides (TMDCs) have received considerable interest in both industrial and biomedical applications.Typically, the formula of a TMDC is MX 2 , where M is transition metal from groups IV, V, or VI, and X is a chalcogen such as S, Se, or Te.Among TMDCs, molybdenum disulfide (MoS 2 ) is the most important 2D nanomaterial due to its tunable band gap 1 and high electron mobility, 2 which make MoS 2 a promising photocatalyst, electrocatalyst, biosensor, etc.In general, MoS 2 nanosheets exhibit three main structural phases: the tetragonal (1T) phase, hexagonal (2H) phase, and rhombohedral (3R) phase, where the 2H and 3R phases are semiconducting, and the 1T phase is metallic. 3Despite the utility of MoS 2 nanosheets in practical applications, the chemical stability of the MoS 2 nanosheets will determine their long-term applications.Prior studies probed the interaction between MoS 2 nanosheets and environmental media, which affected the fate and transport of the MoS 2 nanosheets.For instance, MoS 2 nanosheets were oxidatively dissolved under alkaline 4 and aerobic 5 conditions.Our previous work demonstrated that the dissolution of chemically exfoliated MoS 2 nanosheets (ceMoS 2 ) was slowed in the presence of natural organic matter (NOM), including Suwannee River natural organic matter (SRNOM) and Aldrich humic acid (ALHA), in dark ambient conditions, while aging of MoS 2 with co-occurring ALHA was accelerated by exposure to sunlight. 6These findings indicated the stability and behavior of MoS 2 nanosheets in aqueous environments and were used to evaluate their persistence in the intended applications.
The interactions of MoS 2 nanosheets with ionic species alter the characteristics and fate of the MoS 2 .For example, Li et al. 7 showed that aggregation of MoS 2 nanosheets dispersed by sodium cholate followed the 2D Schulze−Hardy rule, and the critical coagulation concentration (CCC) of MoS 2 nanosheets was smaller in the presence of higher valence cations.Similarly, the CCC of MoS 2 exposed to various cations decreased in the order K + > Mg 2+ > Al 3+ . 8In the presence of a natural macromolecule (e.g., NOM), the aggregation rate of MoS 2 nanosheets was drastically reduced even with high ionic strengths (ISs). 9Liu et al. 10 suggested that higher valence cations suppressed the electrical double layer for MoS 2 nanosheets, and aggregation of the MoS 2 nanosheets was accelerated by Ca 2+ under visible light irradiation. 10In the chemisorption of cations, the sulfur atoms on MoS 2 are soft Lewis base sites with strong affinities for soft Lewis acids (e.g., Hg 2+ and Ag + ). 11Mi and coauthors demonstrated that MoS 2 nanosheets reduced heavy metal ions that had higher reduction potentials than MoO 4 2− and SO 4 2− /MoS 2 pair (0.429 V) (e.g., Ag + (0.7996 V), Hg 2+ (0.920 V), and Cr(VI) (1.232 V)) and released soluble MoO 4 2− and SO 4 2− . 12,13These findings indicated that charge transfer between MoS 2 and the cationic species altered the physical and chemical properties of MoS 2 .Nevertheless, the transformations undergone by MoS 2 nanosheets during interaction with anionic environmental species remain largely unexplored thus far.Marks et al. 14 illustrated the negative impact on the stability of MoS 2 in the presence of model oxidants NO 2 − and BrO 3 − , while dissolved oxygen is a prerequisite for the reaction.The prevalence of anions and their potential interplay with MoS 2 have prompted exploration of the impacts of anionic species on MoS 2 in aquatic systems.
Inorganic anionic species are ubiquitous in aqueous environments, and the concentration profiles of these anions vary with the surrounding geology, ecology, and human activities.For example, the chloride (Cl − ) concentration in shallow groundwater has increased from 0.02 to 0.34 mM to several to tens of mM due to human activities. 15Fertigated water contains 1.36−4.43mM nitrate (NO 3 − ), 16 and sulfate (SO 4 2− ) concentrations of 12.09 mM originating from mining activities were found in rivers. 17The bicarbonate (HCO 3 − ) present in natural water and wastewater comes from dissolved carbon dioxide in the atmosphere, with concentrations ranging from 1 to 5 mM. 18,19The concentration of dissolved phosphate is typically low due to its low mobility, 20 but in swine wastewater, the total phosphorus concentration can be as high as 19.37−45.21mM. 21The effluent from wastewater treatment plants (WWTPs) contains 1.30−3.36mM Cl − and 0.23−1.01mM NO 3 − , 16 and the SO 4 2− concentrations in industrial effluents are 2.60−5.21mM in most countries. 22,23herefore, the engineered nanomaterials (ENMs) released or applied in water treatment facilities inevitably contact anionic species, and the efficacy of their performance could be altered by the presence of anions.Jeong et al. 24  , and H 2 PO 4 − in graphene oxide (GO)-based composites. 25−27 Additionally, the photocatalytic degradation of contaminants has been affected by anions.In a study by Lien et al. 28 , bromide ion (Br − ) promoted the photocatalytic degradation of sulfamethoxazole with CaCu 3 Ti 4 O 7 perovskite due to the formation of highly reactive radicals, while Cl − decreased the degradation rate.Another study by Chen and Liu showed that higher valence anions suppressed the photodegradation efficiency, and the effect decreased in the order PO 4 3− > SO 4 2− > NO 3 − . 29Evidently, the effects of anions in aqueous solutions cannot be overlooked, and further research is required to fully comprehend the role of anions on ENMs.Furthermore, anions may also alter the surface characteristics and chemical stabilities of ENMs.For instance, the surfaces of silver nanoparticles (AgNPs) were chlorinated by chloride or sulfided by sulfide. 30Levard et al. 31 reported that AgNPs formed solid AgCl (s) at low Cl/Ag ratios (≤2675), whereas at Cl/Ag ratios ≥2675, the formation of soluble AgCl x (x−1) led to dissolution of the AgNPs. 31Liu et al. 32 demonstrated that the sulfidation of AgNPs requires O 2 .They also illustrated that direct sulfidation occurred at high sulfide levels (≥0.025 mg/ L), whereas at low sulfide levels (≤0.025 mg/L), intermediate Ag + species were the predominant oxidized form.A study of environmental anions by Guo et al. showed that among environmental anions, only sulfide inhibited the dissolution of AgNPs and alleviated their toxicities, but the release of Ag + was not affected by phosphate or chloride. 33Overall, the impacts of anions on the transformations and intended applications of ENMs have been recognized; therefore, the effects of anionic species on MoS 2 nanosheets need to be further elucidated.
Given that MoS 2 -based nanosheets are promising membrane materials for water treatment and purification, including heavy metal removal, 34 dye rejection, 35 and desalination, 36 it is crucial to assess the chemical stabilities of MoS 2 in aquatic environments with coexisting species, including anions.However, the effects of anions on MoS 2 remain largely unknown.Thus, the aims of the present study were to elucidate the chemical stability of MoS 2 nanosheets with coexisting environmental anions (e.g., Cl − , NO ) under dark ambient conditions and during irradiation with sunlight in aqueous environments.The effects of the anions on MoS 2 were determined by electrochemical analyses and X-ray photoelectron spectroscopy (XPS).Our findings suggest that the presence of anionic species affected the chemical stability of the MoS 2 nanosheets, which will enable evaluations of the roles of inorganic anions in the environmental transformations of MoS 2 .

Characterization and Electrochemical
Measurements of ceMoS 2 .The chemicals and synthesis of ceMoS 2 are described in the Supporting Information (Text S1 and Figure S1).The preparation of the chemically exfoliated MoS 2 nanosheet solutions (ceMoS 2 ) followed previous studies with minor adjustments. 6The morphology of ceMoS 2 was surveyed by high-resolution transmission electron microscopy (TEM) (JEOL JEM-1200).The optical absorption spectrum of ceMoS 2 was determined with a spectrophotometer (HITACHI U-3900).The concentration of the as-prepared ceMoS 2 was determined from the absorbance at 450 nm with a mass extinction coefficient of 5010 L m −1 g −1 (Figure S2). 6The surfaces of ceMoS 2 were analyzed with XPS (ULVAC-PHI, PHI 5000 VersaProbe/Scanning ESCA Microprobe).The binding energies were calibrated with the C 1s peak at 284.6 eV.The Mo 3d and S 2p core-level XPS data were analyzed with XPSPEAK41 software by using Gaussian−Lorentzian components after Shirley background subtraction.The hydrodynamic radius (R h ) and zeta potential of ceMoS 2 suspensions were measured by a ZetaSizer Nano ZS (Malvern Instrument, Worcestershire, U.K.) with a monochromatic coherent 633 nm He−Ne laser.Electron paramagnetic resonance (EPR) spectroscopy (Bruker EMX-10/12 EPR spectrometer) was applied to monitor radicals (e.g., •OH and NO 2 •) generated from ceMoS 2 or/and anions under a light source within the solar range (MORITEX, Hg lamp, 150 W). 5,5-Dimethyl-1pyrroline-N-oxide (DMPO) was adopted as the spin-trapping agent.EPR signals were recorded at 298 K with a microwave power of 40 mW, power attenuation of 7 dB, modulation frequency of 100.0 kHz, and modulation amplitude of 1.0 G.The electrochemical investigations were conducted with an electrochemical workstation (CH Instruments, Inc.) with a Environmental Science & Technology three-electrode system.The glassy carbon working electrode was modified by drip-casting 10 μL of 100 mg/L ceMoS 2 and drying at room temperature.The reference electrodes were Ag/AgCl or saturated calomel electrodes (SCE), and the counter electrode was a platinum wire.The measured potentials were converted to the reversible hydrogen electrode (RHE) potential.The probed anions (Cl − , NO  S3a).Throughout irradiation, the irradiated samples were maintained at 25 °C in a recirculating water bath.To probe the wavelength dependency, the effects of irradiated nitrate ions were characterized with visible wavelength illumination.ceMoS 2 (10.5 mg/L) mixed with 10 mM NaNO 3 was irradiated in a sunlit simulator equipped with a UV-cutoff filter that blocked irradiation with wavelengths below 420 nm (Figure S3b).For a primary assessment, the metrics of the reaction were monitored, including the absorbance at 450 nm (Abs 450 ) and the pH.To further determine the effects of anions on ceMoS 2 dissolution, the dissolved Mo species were gathered by filtering and centrifuging with ultrafiltration tubes (Amicon Ultra15 3 kDa, Millipore, USA).The concentrations of dissolved Mo species in the filtrate were digested and then determined with inductively coupled plasma−optical emission spectrometry (ICP−OES) (PerkinElmer Optima 8000).A control test indicated that there is no adsorption loss of Mo ionic species to the utilized 3 kDa MWCO membranes by comparing the concentration of sodium molybdate solution and its filtrate under different pH values and in the presence of anionic species (Figure S4).The IS effect on the dissolution of ceMoS 2 was examined with 1, 10, and 100 mM anionic species (Cl − , NO 3 − , SO 4 2− , HCO 3 − , and HPO 4 It is worth noting that, except for the sets with 100 mM anions, the IS of the ionic species concentrations used in this study (Table S1) were lower than the CCC (50 mM of KCl) 37 of ceMoS 2 .Additionally, these concentrations were also lower than the minimum level (31.6 mM of KCl) 38 known to affect the transport of ceMoS 2 .− ) vary from a few μM to hundreds of mM. 15,17,39lthough environmentally relevant concentrations of MoS 2 are unknown, the rapid development and widespread use of MoS 2 nanosheets has prompted the need to evaluate the transformations of MoS 2 nanosheets.According to a prior study by Surette et al. 40 , the environmental concentrations of ENMs ranged from a few ng/L to a few mg/L, with lower concentrations considered to be more realistic.Thus, the experimental concentrations of the anions and ceMoS 2 were 1 mM and 100 μg/L, respectively.The dissolved Mo species were collected and measured with high-resolution inductively coupled plasma−mass spectrometry (HR-ICP−MS) (Thermo Scientific Element 2), which can quantify ultralow concentrations (ng/L).

Characterization of ceMoS 2 .
The as-prepared ceMoS 2 was characterized with TEM, UV−vis spectrophotometry, and XPS (Figure 1).The TEM image of ceMoS 2 (Figure 1a) showed that ceMoS 2 had a sheet-like appearance with lateral sizes of approximately 200−300 nm, which was consistent with the general morphology of ceMoS 2 . 4In the UV−vis spectrum (Figure 1b), ceMoS 2 exhibited no peaks in the visible region, which was attributed to the metallic nature of the 1T phase, 41 the dominant phase in the chemically exfoliated (i.e., lithium-intercalated) MoS 2 nanosheets.The orbital configuration and phase composition of ceMoS 2 were analyzed by XPS.In Figure 1c, the Mo 3d doublets of ceMoS 2 were located at approximately 229.0 eV (Mo 3d 5/2 ).After peak fitting, the Mo 3d peaks of ceMoS 2 were deconvoluted into the 1T phase (3d 5/2 : 228.2 eV, 3d 3/2 : 231.3 eV) and 2H phase (3d 5/2 : 229.1 eV, 3d 3/2 : 232.2 eV), which indicated the different binding energies (0.7−0.9 eV) of the 1T and 2H phases. 42The 1T phase content in the Mo−S bond was 64.7%, confirming the predominance of the 1T phase in ceMoS 2 .In the S 2p core-level spectrum (Figure 1d), the broad peak was partitioned into four peaks corresponding to the 1T phase (2p 3/2 : 161.1 eV, 2p 1/2 : 162.8 eV) and the 2H phase (2p 3/2 : 162.0 eV, 2p 1/2 : 163.5 eV).The content of the 1T phase indicated by the S 2p XPS data was 64.1%, which was similar to the result from the Mo 3d XPS data.The laminate structure and the presence of the 1T phase in the as-prepared ceMoS 2 implied a successful synthesis of chemically exfoliated MoS 2 .

Effect of Anions on the Suspension Stability and Dissolution of ceMoS 2 .
The effects of anions on the chemical stability of ceMoS 2 were probed in the presence of 1−20 mM NaCl, considering that Cl − is one of the most common anions in both natural water 43 and wastewater. 44iven the correlation between the absorbance at 450 nm and the concentration of suspended MoS 2 (Figure S2), the changes in the concentration of MoS 2 nanosheets were tracked with the normalized absorbance A t /A 0 , where A t and A 0 are the 450 nm absorbance of the ceMoS 2 dispersion at time t and the initial point, respectively.As shown in Figure S5, a slight decrease in A t /A 0 was observed upon increasing the Cl − concentration from 1 to 20 mM under dark conditions.During exposure to sunlight, the decrease in A t /A 0 for ceMoS 2 became pronounced at higher chloride concentrations (10 and 20 mM), which were 2.0 and 2.8 times greater than that of the control at 48 h, indicating light-accelerated destabilization of the ceMoS 2 dispersion.The light-induced destabilization of MoS 2 near the surface of the sunlit water was ascribed to the surface plasmon oscillations enhanced by the cations (e.g., Na + ), 10 which decreased the energy barrier for aggregation.
Additionally, the concentration of dissolved Mo species produced after 72 h was determined by passing the suspensions through 3 kDa membranes.The dissolution rate of ceMoS 2 increased from 29.1% (0 mM Cl − ) to 35.2% (1 mM Cl − ), 40.1% (5 mM Cl − ), 42.9% (10 mM Cl − ), and 54.3% (20 mM Cl − ) in the dark, and the dissolution rates were increased to 47.4% (0 mM Cl − ) and 68.0%(20 mM Cl − ) with light exposure.The concurrent dissolution of Mo species and the decrease in A t /A 0 suggested that the destabilization of ceMoS 2 with Cl − and light was due to not only light-and cationinduced aggregation but also oxidative dissolution of the ceMoS 2 .Furthermore, the effects of different anions on the transformations of ceMoS 2 and the role of sunlight have not been well-explored previously.Therefore, the transformations of ceMoS 2 dispersions were examined with various anions (i.e., Cl − , SO H 2 PO 4 − under both dark and irradiation conditions.Notably, with the elimination of O 2 , the stabilities of ceMoS 2 suspensions in the presence of the probed anions exhibited no significant difference to the blank control (Figure S6), illustrating the essential role of O 2 in the promoted decrease in − and HCO 3 − , respectively.To determine the oxidative transformation of ceMoS 2 caused by anions, the products from ceMoS 2 were identified by filtering the dispersions through 3 kDa MWCO membranes, and the dissolved Mo species from ceMoS 2 were monitored after incubation with anions.As shown in Figure 2b, the amount of ionic Mo species produced (C t /C 0 ) was computed from the normalized concentration of ionic Mo species at a predetermined time (C t ) and the initial concentration of dissolved Mo species (C 0 ) (i.e., 2.65 mg/L).The produced ionic Mo species displayed a similar trend for A t /A 0 and illustrated that HCO 3 − and HPO 4 2− /H 2 PO 4 − facilitated the formation of dissolved ionic species from ceMoS 2 under both dark and irradiated conditions.Note that the MoS 2 nanosheets can produce photogenerated free radicals (e.g., •O 2 − and •OH), leading to facilitated oxidative dissolution of MoS 2 with light exposure. 5Furthermore, bicarbonate and phosphate promoted the detachment of photogenerated reactive radicals from the surfaces of materials, thereby increasing the mobility of radicals and enhancing the oxidation reaction. 45,46As shown in Figure S7, photogenerated •OH was identified in the irradiated ceMoS 2 suspension in the presence of HCO /H 2 PO 4 − under irradiation was attributed to the enhanced availability of the photogenerated active species.
In the presence of a higher concentration of anions (10 mM in Figure 2c), A t /A 0 showed a greater decline rate compared to those seen at 1 mM, particularly under irradiation, which was due to the destabilization of ceMoS 2 during irradiation in concentrated electrolyte solutions.The light-induced destabilization of ceMoS 2 was further verified by the zeta potential of ceMoS 2 suspensions (Figure S8), which illustrates that the zeta potential of irradiated ceMoS 2 with anionic species became less negative (i.e., destabilization) as a function of time, while the zeta potential remained relatively unchanged under dark condition.In Table 1, the amount of ionic Mo species produced (C t /C 0 ) was further computed from the normalized concentration of ionic Mo species at longer term for 72 h (C t ) and the initial concentration of dissolved Mo species (C 0 ).In the presence of HCO 3 − and HPO 4 2− /H 2 PO 4 − , the production of ionic Mo species from ceMoS 2 was significantly elevated under both dark and irradiated conditions.Additionally, greater dissolution of ceMoS 2 was found with NO 3 − and irradiation.The findings were consistent with observed A t /A 0 , demonstrating that the chemical stability of the MoS 2 nanosheets was affected by the different anions in aquatic systems.
With 100 mM anion concentrations (Figure 2e), the A t /A 0 for ceMoS 2 plummeted during the first 12 h, indicating that the suspension stability was disrupted at high anion concentrations.The suspension stability was monitored using timeresolved dynamic light scattering (DLS) to determine the time-dependent change in the hydrodynamic radius (R h ).Although DLS is not ideal for determining nonspherical particles, the intensity averaged R h could be used to obtain a general index of the size population of ceMoS 2 suspensions. 10,47As shown in Figure S9, the significant increase in R h with the presence of 100 mM anionic species confirms the disrupted suspension stability at higher anion concentrations.In the dissolved Mo species measurement, an elevated release of Mo ionic species was seen with 100 mM HCO 3 − and HPO 4 2− /H 2 PO 4 − and was increased by 129.3% with NO 3 − and irradiation for 48 h (Figure 2f).The nitrate-enhanced transformations of ceMoS 2 seen during solar irradiation are discussed later.Collectively, the findings revealed that oxidative transformation of the ceMoS 2 was promoted to different degrees by environmental anions; HCO 3 − and HPO 4 2− /H 2 PO 4 − promoted the degradation of ceMoS 2 into dissolved Mo species under both dark and irradiated conditions, while NO 3 − exhibited light-accelerated dissolution of ceMoS 2 at higher concentrations.
It is worth noting that the trend for oxidative dissolution was clear in the dissolved Mo species measurements, while A t /A 0 determined the concentration of the MoS 2 dispersion, which was affected by aggregation and sedimentation of the ceMoS 2 nanosheets.As listed in Table S2, the time-dependent A t /A 0 in 1, 10, and 100 mM anionic species in Figure 2 was fit by the first-order decay equation, which illustrates an increase in rate constants under higher anion concentrations (e.g., 100 mM) and light exposure.Given that the aggregation and sedimentation of the ceMoS 2 nanosheets contribute to the normalized absorbance A t /A 0 , 10 the results, in addition to the zeta potential (Figure S8) and DLS measurement (Figure S9), clearly indicated a light-and higher-anion-concentrationinduced destabilization of ceMoS 2 .−50 As shown in Figure 2b,d, the

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Mo species produced in ceMoS 2 were comparable with 1 and 10 mM anionic species, while ceMoS 2 with 100 mM anionic species demonstrated a higher initial dissolution rate (Figure 2f).Along with the potential influence of aggregation on ceMoS 2 dissolution at higher anion concentrations, the promoted oxidative dissolution of ceMoS 2 by HCO 3 − and HPO 4 2− /H 2 PO 4 − , as well as irradiated NO 3 − , was consistent in the probed anion concentration range (i.e., 1−100 mM).
Most of the concentrations of ionic Mo species were higher in the sunlight-irradiated samples than in the dark samples (Table 1), while irradiation with NO 3 − produced more dissolved Mo species (C t /C 0 = 2.37).The enhancement of ceMoS 2 dissolution by NO 3 − and sunlight was attributed to photolysis of the nitrate ions and the generation of hydroxyl radicals: 51,52 (2) EPR analysis was employed to determine the photoproduced radicals.In Figure S10a, the photogenerated •OH and NO 2 • in NO 3 − were detected by distinguishable EPR signals of 2-hydroxy-5,5-dimethyl-1-pyrrolidinyloxy (DMPO− OH) 53 and 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX) 54 (Figure S10b) adducts, respectively.Notably, the oxidation of ceMoS 2 by nitrate was more pronounced under sunlight irradiation.In a comparison experiment, the C t /C 0 of Mo ionic species with NO 3 − and visible light irradiation (with UV-cutoff filter (<420 nm)) was 1.55, which was much lower than that seen for full-spectrum sunlight irradiation (i.e., C t /C 0 = 2.37).This was consistent with the fact that NO 3 − absorbs light below 350 nm (Figure S11) in the solar irradiation, 51 while other anions absorbed no UV and were optically transparent.Scheme 1 depicts the UV-accelerated oxidation and dissolution of ceMoS 2 with nitrate anions.
Given that the greatest oxidative dissolution of ceMoS 2 was found in the presence of HCO 3 − , the ionic Mo species produced in the oxidative dissolution process were examined by ion chromatography.As illustrated in Figure S12, the MoO 4 2− and SO 4 2− produced indicated that aging of the ceMoS 2 in the presence of HCO 3 − occurred according to the reported dissolution reaction of MoS 2 (eq 4), 4 which suggested that oxidative dissolution, rather than the complexation of anion by the ionic Mo species, was likely the dominant reaction pathway.
Previously, NO 2 − and BrO 3 − acted as the oxidants for promoting the dissolution of MoS 2 in the presence of dissolved oxygen, owing to their relative ease of reduction.Given that the reduction potential of the probed anions in the current work is generally more negative (see Table S3) than that of MoO 4 2− and SO 4 2− /MoS 2 pair (0.429 V), 12 the probed anions herein were likely not acting as oxidants.While the essential role of O 2 in this oxidative dissolution has been illustrated (Figure S6), the promoted oxidative dissolution of ceMoS 2 by HCO 3 − and HPO 4 2− /H 2 PO 4 − is surprising since they are unlikely to act as oxidants, thereby further investigation through electrochemical characterization will be conducted later to unveil the origin of the enhanced oxidation.Note that the ionic radius of SO 4 2− and H 2 PO 4 − are larger than Cl − , NO 3 − , HPO 4 − , and HCO 3 − (Table S4), which results in no identifiable correlation with the observed trend in the oxidative dissolution of ceMoS 2 and thus suggests that the ionic radius did not likely play a decisive role in affecting the oxidative dissolution of MoS 2 .Additionally, along with the dissolved Mo species, the protons released throughout the aging process were monitored.As demonstrated in Figure S13, the pH of the ceMoS 2 solution decreased from neutral to acidic in the presence of Cl − , NO /H 2 PO 4 − , which was likely due to their buffering capacities.Notably, a pH dependence was previously illustrated for oxidative dissolution of ceMoS 2 , and the dissolution rates were accelerated at higher pH. 4 To examine whether the oxidative dissolution of ceMoS 2 , particularly in the presence of HCO 3 − , resulted solely from pH-induced reactions, the A t /A 0 values for ceMoS 2 with HCO 3 − and ceMoS 2 in the control medium (i.e., no anions) were compared at pH 8.5.In Figure S14, the pronounced A t / A 0 declines seen under both dark and light conditions in the presence of HCO 3 − compared to those in the pH 8.5 control medium, clearly indicated that other factors, in addition to the pH dependency, regulated the oxidative dissolution of MoS 2 .The mechanism for dissolution of ceMoS 2 in the presence of anions is discussed later.

Morphology and Phase Transitions of ceMoS 2
Triggered by Anions.The morphological and phase transitions of ceMoS 2 triggered by anions were analyzed by TEM and XPS.As shown in Figure 3a, no change in the asprepared nanosheets (Figure 1a) was observed after 72 h of dark incubation.In the presence of anions under dark conditions, the structure of the MoS 2 nanosheets deteriorated, and cracks appeared on the surface and edges, particularly in the presence of HCO 3 − (Figure 3h) and HPO 4 3i).The anion-induced morphological deterioration was consistent with the ceMoS 2 dissolution profiles, both of which indicated the decay of ceMoS 2 stability.During irradiation, no significant difference was observed with Cl − (Figure 3e) and SO 4 2− (Figure 3f) relative to their dark controls.In contrast, the sheet edges of ceMoS 2 were destroyed by the presence of NO 3 − (Figure 3j), indicating photoenhanced destruction by nitrate.In the presence of HCO 3 − (Figure 3k), ceMoS 2 was utterly fragmented, which was consistent with the enhanced dissolution of ceMoS 2 during sunlight exposure (Figure 2).
Prior studies showed phase-dependent oxidative dissolution of ceMoS 2 during the aging process; the 1T phase selectively underwent oxidative dissolution, leading to conversion of the 1T phase to the 2H phase. 4,6Herein, the phase transition of ceMoS 2 was monitored in the presence of various anions with high-resolution Mo 3d core-level XPS under both dark and  4 and S15).After a 72-h incubation in the dark (Figure 4a), the 1T content of ceMoS 2 exhibited a slight decline from 64.7% (as-prepared, Figure 1c) to 58.7% in the dark blank and 55.4% under sunlight.In addition to the phase transition, an increase in the Mo VI −O content of ceMoS 2 was observed from 19.8% in the dark to 30.9% under irradiation.In the presence of Cl − , NO 3 − , and SO 4 2− , ceMoS 2 displayed 1T:2H ratios similar to that in the blank, which was consistent with the relatively low dissolution rate of ceMoS 2 in the presence of these three anions.On the other hand, for HCO 3 − and HPO , and new peaks were found at approximately 230 eV (3d 5/2 ) and 233 eV (3d 3/2 ), for pentavalent Mo (Mo V ). 55The origin of the emergence of Mo V with HCO 3 − and HPO 4 2− /H 2 PO 4 − will be discussed later.Under sunlight irradiation for 72 h, the proportion of the 1T phase in ceMoS 2 exposed to all anions was decreased to a lower level compared to that in the dark.In particular, the Mo VI −O proportion reached its highest value (62.0%) in the presence of NO 3 − and sunlight irradiation, suggesting that a photoreaction of NO 3 − with ceMoS 2 caused a greater oxidation of the 1T phase.The photooxidation of ceMoS 2 by NO 3 − was likely due to the formation of reactive radicals that oxidized the MoS 2 nanosheets (Scheme 1).The changes in morphology and phase transition indicated that the stability of ceMoS 2 varied as a function of anion species and light exposure, and detailed mechanisms are discussed in the following section.

Dissolution Mechanism for ceMoS 2 in the Presence of Anions.
To elucidate the interactions between ceMoS 2 and anions, the electrochemistry of ceMoS 2 was

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probed with various approaches, including OCP, CV, and ORR.As described by Lee et al. 56 , during the OCP process, the electrons transferred from the electrolyte charged the electrode surface.Thus, with a shorter time required to achieve a stable potential, electron transfer from the anions to the electrode surface is accelerated.In Figure 5a, the OCP of ceMoS 2 in a 100-mM anion solution was recorded for 2 h to reach a stable potential.As shown in Figure 5b, the OCP stabilization rate of ceMoS 2 was faster with HCO 3 − and HPO 4 2− /H 2 PO 4 − , while the rates with Cl − , SO 4 2− , and NO 3 − were relatively slow.Given that there was no current flowing during OCP measurements (i.e., equilibrium was achieved between the working electrode and the reference electrode in the electrolyte), 57 the OCP change indicated the electron transfer rate from the electrolyte to the electrode surface.Therefore, HCO 3 − and HPO 4 2− /H 2 PO 4 − exhibited faster charging rates with ceMoS 2 , while Cl − , SO 4 2− , and NO 3 − exhibited slower charging rates.Fast electron charging with the HCO 3 − and HPO 4 2− /H 2 PO 4 − was confirmed by the emergence of Mo V (Figure 4e,f), rather than the transition to Mo VI , in the presence of other anions.
Next, the extent of ceMoS 2 oxidation by the different anions was assessed by CV. Figure 5c shows the oxidation sweep starting at 0 V followed by the reduction sweep.The oxidation peaks indicated oxidation of the ceMoS 2 nanosheets, while the reduction peaks were attributed to reduction of the oxidized MoS 2 . 58,59Among the anionic electrolytes, HCO 3 − and HPO 4 2− /H 2 PO 4 − generated higher oxidation peak currents for ceMoS 2 , implying oxidation of more ceMoS 2 .Additionally, the reduction peak currents for ceMoS 2 were negligible in the presence of HCO 3 − and HPO 4 2− /H 2 PO 4 − , which was ascribed to oxidative dissolution of the ceMoS 2 .This led to deterioration of the electrode material (i.e., ceMoS 2 ) and explained the low reduction peak current in the subsequent cathodic scan. 60Lower current intensities for ceMoS 2 were observed in the presence of NO 3 − , SO 4 , and Cl − , suggesting less ceMoS 2 was oxidized.The electrochemical data were consistent with the observed trend for the dissolution of ceMoS 2 in anion solutions under dark conditions (Table 1).
The results of the OCP and CV suggested different charging rates and oxidative dissolution of ceMoS 2 in the presence of the examined anions, which was ascribed to the chemical potential of the ceMoS 2 surface.In a prior study, Lenhart and coauthors 61 indicated that the oxidative dissolution of AgNPs by electron acceptors (e.g., O 2 ) was catalyzed by nucleophilic reagents owing to alteration of the chemical potential of the AgNPs by the nucleophiles.In the absence of nucleophiles, oxidation of the AgNPs shifted the chemical potential of the particle surfaces to more positive values, which decreased the difference in the chemical potential between the AgNPs and the electron acceptors (e.g., O 2 ) and slowed the oxidation.In contrast, in the presence of absorbed nucleophiles, the nucleophilic reagents generated excess negative charge 62 and shifted the chemical potential of the AgNPs to a more negative value, enabling oxidation of the AgNPs.A nucleophile is an electron-rich molecule or ion with a lone pair of electrons, and it reacts with electron-deficient compounds; 63  , Cl − , SO 4 2− , and NO 3 − are 3.8, 3.8, 2.7, 2.5, and 1.0, 64,65 respectively, indicating that HCO 3 − and HPO 4 2− are the strongest nucleophiles among the probed anions.As shown in Figure 5c, the anodic peak potential and current of ceMoS 2 (highlighted zone) in HCO 3 − and HPO 4 2− illustrated an increase in the oxidation propensity of ceMoS 2 .The compiled results suggest that the enhanced oxidative dissolution of ceMoS 2 in HCO 3 − and HPO 4 2− was likely similar to that of the AgNPs, as the presence of nucleophiles shifted the chemical Additionally, the Mo atom was an electron donor, and the S atom was as an electron acceptor in the electronic structure of MoS 2 , 66 which indicated covalent Mo−S bonding and an electron-deficient Mo center.Moreover, the chemical exfoliation and lithiation process induced sulfur vacancies on the MoS 2 nanosheets, 67 thereby rendering the surrounding Mo atoms more electrophilic. 68,69The Mo VI −O peak in the Mo 3d core-level spectrum of ceMoS 2 (Figure 1c) indicated the presence of sulfur vacancies. 70When potent nucleophiles (HCO 3 − and HPO 4 2− /H 2 PO 4 − ) were present, the formation of Mo V in the ceMoS 2 (Figure 4e,f) was the result of reduction of MoS 2 by the nucleophilic anions.The Mo V altered the chemical potential of ceMoS 2 , which was more susceptible to oxidation and subsequent dissolution.
Given that oxidation dissolution of ceMoS 2 with the electron acceptor O 2 was catalyzed by nucleophilic anions and involved the electrophilic Mo center, oxygen reduction during the oxidation of ceMoS 2 was examined by studying the ORR in the presence of different anionic electrolytes.In the ORR polarization curves (Figure 5d), ceMoS 2 exhibited a more positive onset potential (E onset ) (listed in Table S5) and lower Tafel slope (Figure 5e) in HCO .These results demonstrated that the impacts of the anions on the stability of the MoS 2 nanosheets were valid at lower concentrations.

ENVIRONMENTAL IMPLICATIONS
The chemical stability of the MoS 2 nanosheets has a considerable impact on the potential uses of MoS 2 , especially in aqueous environments.Our findings demonstrated that the oxidative dissolution of ceMoS 2 was affected by the nucleophilicities of coexisting anionic species.The ceMoS 2 charging effects of the nucleophilic anions (i.e., HCO >90%) and elevate the concentrations of HCO 3 − in aqueous environments. 72Between 2004 and 2019, the DIC level in the global ocean increased from 16 to 38% at depths shallower than 1500 m. 73 Thus, the reactions of HCO 3 − with released ENMs, including MoS 2 , in aquatic environments hold increasing importance.The effects of anions could also provide insights into the environmental implications of MoS 2 nanosheets, including their toxicity and bioavailability.It has been  shown that the transformed ENMs throughout environmental processes exhibit different toxicity profiles from those of asprepared materials. 74Our prior study demonstrated that acidic ionic Mo species released during the aging process of MoS 2 nanosheets were detrimental to aquatic organisms. 75Given the dependence of ceMoS 2 oxidative dissolution on the anionic species studied in the present work, the ecological risks of the transformed MoS 2 nanosheets could be shifted by these species.Additionally, our findings demonstrated the accelerated dissolution of ceMoS 2 with anions, particularly NO 3 − , HCO 3 − , and HPO 4 2− /H 2 PO 4 − , under sunlight exposure.Therefore, with the numerous photocatalytic applications of MoS 2 in aquatic environments, 76,77 it is important to consider the presence of anionic species when assessing the durability of MoS 2 for photocatalytic water treatment.

■ ASSOCIATED CONTENT
* sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.3c08278.Supplementary experimental section; residual molar ratio of Li to Mo and pH values of ceMoS 2 suspension; correlation between ceMoS 2 concentrations and absorbance at 450 nm; spectra of natural sunlight and light emitted in the CPS+ solar simulator and transmittance spectrum of the UV-cutoff filter; passing ratio of Na 2 MoO 4 through the 3 kDa MWCO membranes; IS calculation of ionic species used in this study; effect of Cl − on the normalized absorbance at 450 nm of ceMoS 2 ; stabilities of ceMoS 2 suspensions in the presence of 1 mM anionic species with and without elimination of O 2 ; EPR spectra for DMPO adducts in ceMoS 2 with HCO 3 − and HPO 4 2− /H 2 PO 4 − ; zeta potential measurement of ceMoS 2 with sodium anionic species; aggregation profiles of ceMoS 2 suspensions with sodium anionic species; rate constants of absorbance in ceMoS 2 with coexisting anionic species; EPR spectra for DMPO adducts in NO 3 − under irradiation; UV−vis absorption spectra of sodium anionic species and the spectrum of light emitted by the CPS+ solar simulator; ion chromatograms of ceMoS 2 incubated with HCO 3 − ; standard reduction potential of anionic species; ionic radius of anionic species used in this study; pH variation of ceMoS 2 incubated with anionic species; pH effects on the stability of ceMoS 2 ; Mo 3d XPS spectra of ceMoS 2 in anions; and the ORR onset potential of ceMoS 2 in anions (PDF)

3 −
demonstrated that the removal of As(V) by Fe 2 O 3 and Al 2 O 3 was not affected by Cl − or NO 3 − but was inhibited by HPO 4 2− due to the structural similarity of arsenate and phosphate.The removal of contaminants was minimally affected by Cl − , NO Relevant Conditions.In natural water, the concentrations of anionic species (Cl − , NO 3 −

/Figure 2 .
Figure 2. Stabilities of ceMoS 2 suspensions (10.5 mg/L) determined by measuring the normalized absorbance at 450 nm (A t /A 0 ) and the normalized concentration of dissolved Mo species (passed through 3 kDa membranes) (C t /C 0 ) in the presence of (a,b) 1 mM, (c,d) 10 mM, and (e,f) 100 mM anionic species under both dark and solar irradiation.Error bars are sample standard deviations from triplicate measurements.
the anions used in this study are nucleophiles with different nucleophilicities.The nucleophilic constants of HCO 3 − , HPO 4 2−

Scheme 2 .
Scheme 2. Proposed Mechanism for Oxidative Dissolution of ceMoS 2 Accelerated by Various Anions

Table 1 .
dissolved Concentrations of Ionic Mo Species (Obtained by Passage through 3 kDa Membranes) in ceMoS 2 (10.5 mg/L) Incubated with 10 mM Anions for 72 h

5. Dissolution of ceMoS 2 under Environmentally Relevant Conditions.
The presence of anions facilitated the oxidation of ceMoS 2 by oxygen by triggering changes in the chemical potential of the ceMoS 2 surface as a function of the nucleophilicities (i.e., charging effect) of the various anions.3.To confirm the effects of anions on the MoS 2 nanosheets, the dissolution of ceMoS 2 in the presence of anions was surveyed under more environmentally relevant conditions.As listed in Table2, the initial concentration of dissolved Mo species from the ceMoS 2 was 12.21 μg/L.After 72 h of incubation in the dark, the C t /C 0 of ceMoS 2 increased to 2.62, 2.76, and 2.57 for Cl − , SO 4 2− , and NO 3 − , respectively, indicating mild enhancement of ceMoS 2 dissolution by these three ions.On the other hand, the C t /C 0 for ceMoS 2 increased substantially to 3.97 and 4.02 in the presence of HCO 3 C t (μg/L) C t /C 0 C t (μg/L) C t /C 0 blank 12.21 ± 0.31 26.76 ± 2.07 2.19 ± 0.22 43.09 ± 1.38 3.53 ± 0.20 Cl − 32.05 ± 1.96 2.62 ± 0.23 43.29 ± 2.16 3.55 ± 0.33 SO 4